Hot gas path components of gas turbines typically employ air convection and air film techniques for cooling surfaces exposed to high temperatures. High pressure air is conventionally bled from the compressor and the energy of compressing the air is lost after the air is used for cooling. Steam-cooling of hot gas path components has been proposed, utilizing available steam from, for example, the heat recovery steam generator and/or steam turbine components of a combined cycle power plant. Where steam is utilized as the coolant for gas turbine components, there is typically a net efficiency gain inasmuch as the gains realized by not extracting compressor bleed air for cooling purposes (typically in an open cycle configuration) more than offset the losses associated with the use of steam as a coolant instead of providing energy to drive the steam turbine. Steam-cooling is even more advantageous when the steam coolant is provided in a closed circuit whereby the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine.
Because of the differences in heat transfer characteristics between air and steam, it would be expected that turbine components designed to utilize these cooling mediums would be constructed differently. For example, turbine buckets designed to be cooled by open circuit air cooling would be expected to be substantially different from turbine buckets designed for cooling by closed circuit steam. In the case of steam-cooling, the coolant would be recovered from the turbine buckets to provide useful work elsewhere. In the case of air cooling, the air would typically be discharged from the buckets to the hot gas path. The internal passages which provide the cooling circuits for the rotational hot gas path components would typically be expected to be designed differently.
For a gas turbine to have the flexibility to be cooled with either air or steam (a feature of the present invention described below), it is necessary that cooling circuits be designed to accommodate both cooling mediums. A customer purchasing a simple cycle gas turbine power plant, for example, would need to have the turbine components cooled by air, if there was no available source of alternative coolant. If, however, the customer later expands his plant to an upgraded combined cycle plant, steam becomes readily available as a coolant and it would be advantageous from an efficiency point of view to utilize this steam to cool the turbine. Consequently, a problem to be addressed in the present application is the provision of cooling circuits in the rotational components of a gas turbine which can readily be converted for cooling such components by using either air or steam as the cooling medium.